The invention relates to the field of fiber-optic pressure and temperature measurement.
In oil production, drill holes have to be monitored with regard to pressure and temperature. The liquid pressures in the drill hole can be up to 100 NPa (1000 bar) , and the temperatures can be up to over 200xc2x0 C. Electric sensors such as, for example, piezoelectric resistors, piezoelectric elements, capacitive probes, or crystal resonators are frequently used in pressure measurement up to approximately 170xc2x0 C. It is also known to use optical pressure sensors which are distinguished by good high temperature capacity, corrosion resistance and insensitivity to electromagnetic interference. Examples of this are mechanical resonators, which are activated optically and read out optically, elastooptic sensors, optical sensors with a pressure-sensitive diaphragm, or Fabry- Perot resonators.
A further optical sensor with fiber Bragg gratings for measuring material elongations is disclosed, for example, in U.S. Pat. No. 4,761,073. A refractive index grating, which is written by UV light into a monomode fiber, acts as a reflector or transmission filter with a characteristic Bragg wavelength xcexB. Longitudinal fiber elongations change the grating period and refractive index and displace the Bragg wavelength xcexB. The output signals are wavelength-coded and independent of the received light power. Serial multiplexing of several elongation sensors can be realized very easily by writing in along a sensor fiber several Bragg gratings with different reflection wavelengths whose signals can be separated spectrally. It is proposed to eliminate signal interference based on thermal grating elongations with the aid of superimposed gratings of different reflection wavelengths. It is known that instead of being done in a wavelength-selective fashion it is also possible for multiplexing to be performed by time-resolved measurements with the aid of a pulsed light source. In order to monitor body deformations, the sensor fiber is typically fastened on the surface of the body or embedded in the body. If Bragg gratings are used for elongation measurements, the measurement range is limited by the ultimate fiber strength.
Fiber Bragg grating sensors for measuring isotropic pressures of liquids are presented in the article by M.G. Xu et al., xe2x80x9cOptical In-Fibre Grating High Pressure Sensorxe2x80x9d, Electronics Letters 29 (4), 398-399 (1993). The sensor fiber is introduced with the Bragg grating into a high-pressure vessel and immediately exposed to the hydrostatic pressure of a fluid. However, the isotropic pressure sensitivity is exceptionally low for Bragg gratings in glass fibers; the specific Bragg wavelength displacement is typically only 0.0003 nm/ 100 kPa at 1550 nm. Moreover, it is necessary to compensate temperature effects because of the high temperature sensitivity of typically 0.01 nm/xc2x0C.
An apparatus for longitudinal compression of optical fibers is described in U.S. Pat. No. 5,469,520. The sensor fiber is threaded with the fiber Bragg grating into several cylindrical ferrules and two end tubelets, and the ferrules and tubelets are mounted in a groove between two metal blocks which can be screwed to one another. The ferrules can be displaced laterally in the groove, one tubelet is connected to the metal blocks and the other is fastened on a moveable slide. By displacing the slide, the fiber is compressed on the free links between the tubelets, in particular between the ferrules, and lateral escape is simultaneously prevented by the groove. It is possible to realize a very wide pressure measurement range because of the fact that the pressure strength of glass fibers (xe2x80x9cfused silica fibersxe2x80x9d) is approximately 20 times greater than their elongation strength.
U.S. Pat. No. 5,042,898 discloses a device for temperature stabilization of fiber Bragg gratings. The fiber Bragg grating is clamped over a gap between two supports with different coefficients of thermal expansion. The supports are screwed to one another at a common supporting point via a spacer thread with the aid of which the gap width and/or fiber prestressing and/or Bragg wavelength can be set. The differential fiber elongation between the fiber holders is dimensioned precisely such that the thermally induced changes in the Bragg wavelength can be compensated. This is achieved by selecting the support materials and the spacings between the supporting point and the fiber holders. In an embodiment which can be subjected to pressure, a glass capillary is provided in the gap for the purpose of accommodating the fiber Bragg grating. The carrier materials and the length and inside and outside diameters of the glass capillary are to be coordinated with one another for the purpose of temperature compensation. A temperature-stabilized fiber Bragg grating of this type can be used as a wavelength standard, for stabilizing the emission wavelength of laser diodes, or as a wavelength filter in fiber-optic sensors.
It is the object of the present invention to specify a fiber Bragg grating pressure sensor which is suitable for wavelength-coded measurement of isotropic pressures in liquids or gases and is distinguished by a compact transducer which can be designed for high pressures. This object is achieved according to the invention by means of the features of claim 1.
Specifically, the core of the invention is to specify a fiber-optic transducer in which a pressure sensor fiber with a fiber Bragg grating is fastened on supports by means of fiber holders, and at least one support is fitted with a pressure member for converting an all round pressure of a medium into a longitudinal elongation of the pressure sensor fiber.
A first exemplary embodiment shows a pressure-transmitting element (=transducer) with a pressure-loaded inner cylinder and an unloaded outer cylinder, which are arranged in a housing whose differential pressure elongation is transmitted to a sensor fiber, and whose differential temperature elongation stabilized the Bragg wavelength of the sensor fiber.
A second exemplary embodiment relates to variants of the transducer in the case of which the outer cylinder is simultaneously the housing and is subjected to pressure, and in the case of which the sensor fiber can also be placed under compressive load.
A third exemplary embodiment constitutes a transducer with an annular cylinder which is pressure loaded from inside, and force-transmitting center cylinders for elongation of the sensor fiber at both ends.
A fourth exemplary embodiment constitutes a transducer with a supporting cylinder which is pressure loaded from outside, and center cylinders, which are oppositely pressure-loaded for the purpose of relieving pressure from the sensor fiber at both ends.
Further exemplary embodiments relate to fiber holders and ferrules for fixing and prestressing the pressure sensor fiber in the transducer.
Another exemplary embodiment constitutes a serial, reflexive multiplexing arrangement of several fiber Bragg grating pressure sensors with different Bragg wavelengths, which are fed via a common broadband light source and are detected in a wavelength-selective fashion.
Additional exemplary embodiments follow by combining features which are essential to the invention, and from the dependent claims.
An important advantage of the fiber Bragg grating pressure sensor according to the invention consists in that it is possible with the aid of the wavelength-coded pressure signal to realize a high measuring accuracy, a wide pressure measurement range of up to 100 MBa and a large measuring distance between the passive sensor head and active optical system and electronic system.
A further advantage of the fiber Bragg grating pressure sensor consists in that it is possible for the temperature sensitivity to be largely suppressed by a differential design of the transducer, and thereby for the reliability of (quasi)static pressure measurements to be substantially improved.
Finally, also very advantageous features of the fiber Bragg grating pressure sensor are the ease with which it can be combined with a fiber Bragg grating temperature sensor, the simple ability to multiplex several combined pressure and temperature sensors and, overall, the excellent suitability for use under high pressures, at high temperatures and with strong corrosion, in particular in oil drill holes.